Abstract
The effects of dissolved solid concentration on the freezing and thawing treatment of excess activated sludge were investigated in this paper. The floc properties and the solid-liquid separation characteristics of the unfrozen and frozen excess activated sludges in which various amount of sodium chloride (NaCl) were dissolved were measured. The moving and concentrating characteristics of suspended solids during the freezing process deteriorated with increasing NaCl concentration, and the size and density of the sludge floc produced became smaller. Settling and filtration experiments showed that the settling and filtration characteristics of the sludge under 0.2 wt% NaCl were remarkably improved by freezing and thawing treatment, and did not depend on NaCl concentration. However, if the NaCl concentration was over 0.2 wt%, the improvement of the settling and filtration characteristics decreased greatly. Therefore, for excess activated sludge in which the dissolved solid concentration is over 0.2 wt%, it is desirable that the freezing and thawing treatment should be done after washing the sludge with water to decrease its concentration.
